Hydraulic circuits and systems have been in use for decades and are often selected because of their "controllability," flexibility of design, and ease of installation and maintenance. Unlike mechanical drive trains, a hydraulic system is not bound by rigid shafts, gears and the like and can be used in applications where other types of drives would, at the least, be impractical.
A basic hydraulic circuit has a reservoir or tank holding hydraulic fluid and a source of pressurized fluid, i.e., a pump, driven by some sort of prime mover. Electric motors and internal combustion engines are common prime movers. And in a hydraulic boat steering system (where the pump is known as a helm pump and is attached to the boat steering wheel), the prime mover is the human operator manipulating such steering wheel.
A hydraulic circuit also has what may be termed a "work device," i.e., a device which uses pressurized fluid from the pump to produce a useful output, e.g., torque and rotary motion or linear force. Common work devices include hydraulic motors of the rotary or linear type. The latter are often called hydraulic cylinders and are available in single-acting and double-acting configurations. A single-acting cylinder has a single rod extending from and movable with respect to an elongate, tube-like housing. A double-acting cylinder has two rods, one extending from each end of the housing.
Known hydraulic circuits are configured in either of two fundamental types. In one type, known as an open-loop circuit, the pump draws fluid from the tank and delivers it to the motor, usually through a valve. Fluid expelled from the motor is returned to the tank.
In the other type, known as a closed-loop circuit, fluid expelled by the motor is returned directly to the pump rather than to the tank. Since such expelled fluid is expelled at a pressure, such pressure helps urge the fluid into the pump.
An advantage of an open-loop circuit is that the fluid (in which air is often entrained) is allow to "dwell" in the tank and give up air entrained therein. Fluid which is substantially free of entrained air is much preferred in a hydraulic circuit since the presence of air (which, unlike hydraulic fluid, is compressible) can make the circuit "spongy." To put it another way, it is easy to get rid of entrained air when using an open-loop circuit.
In a boat steering system, the helm pump is usually of the piston type because of their inherent higher efficiency and low leakage. In a common type of piston helm pump, there is within the housing an angled swash plate and a barrel with pistons reciprocating therein. Each piston is urged against the swash plate by a separate spring. The barrel is connected to the pump shaft and as the steering wheel is rotated, each piston moves in its bore in a direction to draw fluid into such bore and then moves in a direction to expel such fluid from the bore.
When used in a boat steering system, an open-loop circuit has some disadvantages. The most significant involves the fact that each pump piston must, in turn, "suck" fluid from the tank. The hydraulic line from the tank to the pump inlet port can impose a rather significant pressure drop.
In consequence, the springs urging the pistons against the swash plate must provide a rather high force to overcome such pressure drop and still retain the piston against the swash plate. Heavy, high-force springs require more effort on the steering wheel and make steering difficult.
Given the above, one would naturally conclude that a closed-loop circuit is the right choice for a boat steering system. This would not be an unreasonable conclusion since, because fluid is "forced" into the pump by the fluid-expelling motor, the piston springs can be much lighter and steering is quite easy. But in a boat steering application, closed-loop circuits are not without their problems.
The most significant arises when the circuit is first installed or when service needs to be performed. In either instance, the circuit must be purged of air so that steering is "solid" and responsive, not spongy. And in a closed-loop system, there is no easy way to purge air. This is so since fluid does not return from the motor to the tank where air would otherwise be released.
The patent literature recognizes the problem of air removal from a hydraulic system. U.S. Patent No. Re 33,043 (McBeth--a reissue of U.S. Pat. No. 4,685,293) describes a system for bleeding air from a closed-loop circuit. The system involves a valve with a number of manually-positioned check valves opened and closed in a sequence to pump oil alternately through lines to a tank. There is no suggestion as to how the valve may be "packaged" with the hydraulic pump to reduce plumbing and simplify system installation.
U.S. Pat. No. 2,882,686 (Griffith) shows a valve structure for use when bleeding or filling a closed-circuit hydraulic system. U.S. Pat. No. 4,933,617 (Huber et al.) depicts an open-loop steering system for boats. Significantly, the Huber et al. system uses force-amplifying servo-assisted steering in normal "non-autopilot" operation.
An improved valve which is very easy to use, which avails the industry of the advantages of both open- and closed-loop hydraulic circuits, as needed, and which may be readily manifolded to a hydraulic pump would be an important advance in the art.